In EU Countries, society’s expectations and political decisions have pushed the adoption of ecologically sustainable ways to manage orchards. Nitrogen (N) nutrition is a powerful means of controlling growth and fruiting of trees and guidelines for N management now aim to limit fertiliser applications below threshold values in order to reduce N losses. Increasing the effectiveness of the recycling of N pools available in the orchard is a basic step to reduce external N inputs. The availability of the stable isotope 15N as experimental tool has made possible significant advancements in the knowledge of the fluxes of N in the soil-tree system. Within-tree N sources for vegetative tree growth and reproduction include remobilization of winter stored N (within the tree and between the years) and root-shoot-root N recycling (within the tree and within each year). Nitrogen remobilization from storage is the major source of N in spring, until root uptake becomes predominant. As trees age, relatively more N in new growth is derived from storage and trees become relatively less dependent on root N uptake. Specific amino acids and amides have been identified in the xylem sap of several trees, including apple and cherry, that are considered responsible for remobilization of N compounds in spring. Most evidence has been obtained with relatively young trees grown in pot so there is a need for developing new approaches for quantifying N storage by adult trees in the field. Shoot-root transport of N and subsequent xylem reloading at the root level is a normal feature of vascular plants. While qualitative evidence of this phenomenon are based on detailed analysis of phloem and xylem sap, quantifying reloading N in the xylem was approached by comparing the N fluxes in the xylem with the accumulation of N in tree canopy. Results indicate that recycling of N in the xylem is a mechanism by which plants might regulate N uptake by roots. The adoption of stable isotope techniques in tree physiology has led to significant improvements in our understanding of cycling of N in trees. It is clear that nitrogen has a very high mobility in the tree and that its initial partitioning to a target organ does not necessarily represent its final destination. Increasing the effectiveness of the recycling of the N already present in the orchard is a basic step to reduce external N inputs. The better understanding of the role of internal tree cycling of N allows a fine tuning of N supply to improve its uptake efficiency and reduce potential losses.
Keywords: Nitrogen storage, nitrogen remobilization, nutrient supply, shoot-root-shoot nitrogen recycling, xylem sap composition